FIG. 3 illustrates an example of conventional flyback type DC-DC converters. The shown prior art DC-DC converter 4 comprises a bridge rectifier 2 connected to an AC power source 1 for full-wave rectification of AC voltage E from power source 1; an input smoothing capacitor 3 for smoothing output from bridge rectifier 2 into stable DC output voltage VO to a load 5. Converter 4 further comprises a primary winding 6a of a transformer 6 and a MOS-FET 7 as a switching element connected in series to input smoothing capacitor 3; a secondary winding 6b of transformer 6 connected to a rectifying smoother which has a rectifying diode 8 and an output smoothing capacitor 9; a choke coil 10 of common mode as an inductance element; a detection circuit 11 for picking out DC voltage VDC produced from output smoothing capacitor 9; and a control circuit 13 for producing drive signals VG to regulate on-off period of MOS-FET 7 based on detection signals transmitted from detection circuit 11 through a photo-coupler 12 to control circuit 13. Choke coil 10 has two windings NA and NB wound around a common magnetic core in in-phase or common mode relation to each other to form the inductance element connected in a line between output smoothing capacitor 9 and load 5.
In operation, control circuit 13 produces drive signals VG to gate terminal of MOS-FET 7 to alternately turn MOS-FET 7 on and off, and thereby apply intermittent AC voltage of high frequency on primary winding 6a of transformer 6. Resultant AC voltage is taken out from secondary winding 6b of transformer 6, and further rectified, smoothed and converted into DC voltage VDC through rectifying diode 8 and output smoothing capacitor 9. Detection circuit 11 picks out DC voltage VDC from output smoothing capacitor 9 to produce detection signals to control circuit 13 through photo-coupler 12. Control circuit 13 provides gate terminal of MOS-FET 7 with drive signals VG regulated based on detection signals from detection circuit 11 to appropriately adjust on-off period of MOS-FET 7 so that output smoothing capacitor 9 produces stable DC voltage VDC, removing common mode noise by choke coil 10 to apply DC output voltage VO to load 5 without noises.
In the DC-DC converter 4 shown in FIG. 3, there is provided an overcurrent protector 14 for restricting DC output current IO to load 5 to avoid possible damage to load 5 and converter 4 in case of excessive output current IO flowing through load 5. Overcurrent protector 14 comprises a current detecting resistor 15 connected in series to first winding NA of choke coil 10 for sensing output current IO through load 5 as a voltage value corresponding to the amount of output current IO; a normal power source 16 for producing a reference voltage VR to define a rated value of output current IO flowing through load 5; and an error amplifier 18 as a comparator for comparing detected voltage VL applied to inverted input terminal (−) of error amplifier 18 from current detecting resistor 15 through a limiting resistor 17 with reference voltage VR applied to non-inverted input terminal (+) of error amplifier 18 from normal power source 16. When current detection voltage VLD exceeds reference voltage VR, error amplifier 18 produces an error signal VOC to control circuit 13 through detection circuit 11 and photo-coupler 12 so that control circuit 13 reduces the time width of drive signals VG of high voltage level to gate terminal of MOS-FET 7 and thereby shorten the on period of MOS-FET 7 in order to restrict output current IO passing through load 5. In the DC-DC converter 4 shown in FIG. 3, as current detecting resistor 15 has the own electric resistance for impeding passage of output current IO, it provides electric power loss which deteriorates conversion efficiency of the converter, and moreover causes thermal loss proportional to the square of output current IO. Accordingly, in case current detecting resistor 15 radiates a large amount of heat emitted during the operation, a problem arises in that such heat would make various repressions in mounting current detecting resistor 15 or other peripheral electric elements.
Japanese Patent Disclosure No. 6-269159 shows a switching power source circuit which comprises a choke coil connected in a line between input and output terminals for attenuating ripple noise or spike noise on the line; and a voltage detector connected between both ends of the choke coil for detecting a current value through the choke coil as a voltage drop by resistance component of the choke coil to provide a given output voltage and protect electric elements against excess current without current detecting resistor.
Japanese Patent Disclosure No. 2001-309654 demonstrates a ringing choke converter having a protective function against excess current and resistive against deterioration in conversion efficiency and ignition or combustion of resistor without utilizing an excess current detecting resistor. The ringing choke converter comprises a switching element for converting DC input voltage into constant output voltage by the on-off operation of the switching element; an excess current protector connected to a control terminal of the switching element for ceasing operation of the switching element during the overload; an output capacitor connected to output terminals; a smoothing coil and a smoothing capacitor connected in series to both ends of the output capacitor; a ripple voltage detector for perceiving ripple voltages on the output capacitor; and a photo-coupler connected to the ripple voltage detector for driving the excess current protector based on output signals from the ripple voltage detector.
FIG. 4 illustrates a DC-DC converter of flyback type having the electric structure similar to those of the converters shown in the foregoing references. Unlike the converter shown in FIG. 3 utilizing the common mode choke coil 10, the converter shown in FIG. 4 comprises a low pass filter circuit which includes a normal mode choke coil 19 and a filtering capacitor 20 connected after output smoothing capacitor 9; a first voltage-dividing resistors 21 and 22 connected in parallel to output smoothing capacitor 9 before choke coil 19; a second voltage-dividing resistors 23 and 24 and a normal power source 16 connected in series to each other and connected after choke coil 19 and in parallel to first voltage dividing resistors 21 and 22; and a comparator 25 for comparing a first divided voltage VA on junction between resistors 21 and 22 and a second divided voltage VB on junction between resistors 23 and 24 to produce output signals VOC of high voltage level H to cease the operation of MOS-FET 7 for restriction of output current IO. Other components in FIG. 4 are similar to those in FIG. 3.
The DC-DC converter 4 shown in FIG. 4 is designed to turn MOS-FET 7 on and off to produce AC voltage of high frequency on secondary winding 6b of transformer 6 and then convert it into DC voltage VDC through rectifying diode 8 and output smoothing capacitor 9, however, output smoothing capacitor 9 has an internal impedance which causes ripple components superimposed on resultant DC voltage. Choke coil 19 and filtering capacitor 20 cooperate to form a combined low pass filter circuit which serves to remove ripple components from output voltage VO which is then supplied to load 5 without ripple. Specifically, ripple components on output voltage VO across output smoothing capacitor 9 appear only on both ends of normal mode choke coil 19, and voltage across choke coil 19 naturally involves lots of ripple components which may have a correlation with output current IO. Nevertheless, ripple components are not transmitted to load 5 due to absorption by filtering capacitor 20. On the other hand, choke coil 19 has a conductive winding of the own slight electric resistance which causes a voltage across choke coil 19 in proportion to output current IO through the conductive winding. Therefore, voltage across choke coil 19 means output current IO through load 5 with low power loss.
In this way, the DC-DC converter 4 is advantageous in that normal mode choke coil 19 can effectively diminish normal mode noises such as ripple and spike noises, however, disadvantageously induces a voltage involving many ripple components across choke coil 19. In using such a rectifying smoother 8, 9 of the capacitor input type, the converter 4 is effective for detection of output current IO, however, detection circuit 11 cannot pick out stable values of output current IO because it fluctuates by moving ripple components before filtering capacitor 20 dependently on change in internal impedance, uneven capacitance and drifting thermal characteristics of output smoothing capacitor 9.
An object of the present invention is to provide a DC-DC converter capable of detecting output current through a load with accuracy and law power loss. Another object of the present invention is to provide a DC-DC converter which can achieve the double detection of excessive output current for improvement in safety.